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jralls writes "The Guardian is reporting that a European ion-rocket has taken the last year to reach the moon and is about to enter lunar orbit. Once it slows and gets into a very low orbit, it will probe the surface with x-rays in an effort to solve the long standing puzzle of the moon's origin."

The general idea of a ionization engine is obviously sustained impulse (as previously stated).

Ionizations engines are typically attatched to probes which have initial combustion engines that give them the thrust required to escape the earth's gravity. Once the probes have escaped gravity, the ion engines allow continual thrust that allow (for extremely long range expeditions) extremely high velocity. It is a competing technology with solar sails - which also allows for huge velocities over long distances

It's fairly simple. The ESA doesn't have a rocket capable of putting a payload on a lunar path. AFAIK Arianne is really only good for LEO stuff. The only two countries with lunar capable rockets are I believe the Russians and the USA. Of course, the ultimate lunar capable rocket, the Saturn V, is dead, so all we can do is cross our fingers and hope that NASA will come to its senses and bring back the big dumb booster.

I wonder if we are able to observe this interplanetary tortoise from earth? If it passes the bright side in full moon, we should have quite a clear view of it since it's going so slowly.

I'm curious what make you think it will be going slowly? It will be orbiting the moon at exactly the same speed as any other craft at the same altitude would be orbiting the moon. The type of engine or thrust has nothing at all to do with orbital mechanics.

It's not going slowly, it's going at the same speed anything else in the same orbit would be at. It's taking a long time to reach lunar orbit because it started out in Earth orbit and needs to accelerate quite a bit. It's using an ion engine, so it'll take longer to achieve a given change in velocity. A conventional rocket would achieve the same change in velocity more quickly, but the end result would be basically the same...except that the conventional rocket would be bigger and heavier, and thus more exp

(Slightly off-topic, but fittingly placed among the Star Wars-related posts.)

Anyway, now that this mode of propulsion is being deployed in (or above) the real world, and the private sector is building spaceships, how long, I wonder, will it be before some rich hobbyist builds a functioning TIE Fighter? All the parts are waiting to be assembled, with the possible exception of the small megawatt-class lasers.

It'd be great; get a bunch of rich Star Wars reenactors together with their lovingly assembled sp

By viewing or using light relected from the moon you agree to be bound by the terms of this License Agreement. IF YOU DO NOT AGREE TO THE TERMS OF THIS LICENSE AGREEMENT, LEAVE THE PLANET IMMEDIATELY AND/OR KILL YOURSELF.

"The sun emits X-rays and these are reflected back into space by atoms on the Moon's surface. A magnesium atom will reflect an X-ray in a different way from an iron atom, and Grande's detector can detect these differences.

Flying over the lunar poles, so that it covers the entire Moon as it revolves below, Smart will create strip maps of the surface - and eventually a global map of its composition."

Look like useful data to me if we were in the 'mine the moon' business... maybe in a not so distant future?

At each pole, there are such deep caverans that sun never reaches it. That includes X-rays and normal sun light. That is why it is possible for ice to be there. And yes, it is considered part of the surface.

There are some craters on the south pole of the moon which never get sunlight inside, which makes them a potentially good spot to collect volatile minerals. See more at Space.com's article on the south pole [space.com].

The ultimate speed of ion propulsion is higher than that of chemical propulsion.

But the mass being expelled at high speeds (the ions) is so low, that accelleration is VERY slow. So it takes a long time to get up to speed, but the maximum speed you can theoretically reach is much greater than that of chemical rockets.

Look up any reasonable book on mechanics and you will find a formula for the final velocity of rockets that have a empty mass M, mass of fuel m, and have an exhaust velocity v. The final velocity of the vehicle is...

V = v . ln( (M + m)/M )

In other words ion rockets will beat chemical rockets because they eject their exhaust at a reasonable fraction of c, whereas chemical rockets have exhaust velocities more like velocities we see on earth (e.g. bullets). So chemical rockets need lots of mass, but that's ok because they throw out lots of mass. Trouble getting to space is expensive... each kilo of fuel you put in orbit better be wisely used... so in space ion rockets make sense (apart from the fact you can't use them on Earth anyway... wouldn't be able to lift off even).

> The ultimate speed of ion propulsion is higher than that of> chemical propulsion.

Depending of course on the fixed mass of the spacecraft, vs it's propellant mass, of course. You get more momentum change from given amount of propellant, but if you only had a teaspoon full of propellant, or the spacecraft was exceptionally massive, you wouldn't get more velocity.

> But the mass being expelled at high speeds (the ions) is so> low, that accelleration is VERY slow. So it takes a long> time to get up to speed, but the maximum speed you can> theoretically reach is much greater than that of chemical> rockets.

To expand, the measure of efficiency of a rocket engine is the specific impulse or ISP. It's how much momentum change you get per unit of propellant mass, and the usual unit is seconds (lb-sec/lb). The highest actually-achievable ISP from a chemical rocket is somewhere in the 475 seconds. The Saturn 5 first stage was more like about 350, and monopropellant thrusters used for many satellite propulsion systems is more like 150-180! That means that if you want to change the velocity a lot, you need a whole lot of propellant.

I'm not sure which engine this particular program uses, but the ISP of the typical Xenon ion thruster is something like 1800. So you have to carry fantastically less propellant for a given velocity change, meaning it can weight less at liftoff, meaning you can use a weaker/cheaper booster.

The downside is that you don't get something for nothing. It takes, not surprisingly, a whole lot of electrical power to make it go. So you put in 4000-5000 watts of power, and it only generates.04 lb of thrust -.64 of an ounce, pushing a spacecraft weighing thousands of pounds on the ground. So the acceleration is very small, meaning takes a long time to get going. The other downside is that the Xenon ions, although chemically pretty neutral, shoot out at such high speeds that anything that gets in the exhaust gets eaten away. This may or may not be an issue depending on there you put it relative to the rest of the spacecraft.

There was no extra weight margins on the launch vehicle to do so. However, if you have a more powerful booster (i.e. lots more money), you can then get higher intial speed.

This mission was to prove to EU that their ion engine worked. So they wanted it to keep running for quit some time. NASA did this be creating deep space one, which ran around picking up steam via its ion engine.

Down the road, you can bet that EU will launch a number of deep space probes based on ion engines with high initial speeds.

In addition, their will be a real push for micro sats with ion engines to control them. Makes a lot of sense to send these to other planets. think of 100 small satillites going though out jupitor or saturn planets. Or better yet, small micro sats around Mars providing surface to space communication, pictures, glp, etc. Send about 100 of these to orbit mars and we would have a very through pic of mars, moon, etc.

I bet when this technology becomes the norm for deep space probes they will use the gravitational sling shot effects rather than attach boosters. Its going to take a while for any deep space probe to get to its destination, so why not use free (as in mass) to help you get there?

Bravo! Bravo! Encore!
From now on this line shall be known as my personal motto! It shall be enscribed in the family coat of arms. It shall be engraved in the family ring. It shall greet the trespasser when approaching the gates of the family manor.

What you heard is that the specific impulse is way larger than with chemical, meaning that the exhaust velocity is very high. That means that you have a very efficient means of propulsion, with each particle of exhaust producing more thrust/particle than chemical rockets do. The exhaust being a tenuous gas however, the actual THRUST is very low (and the thrust/weight ratio even more so). Because it works in the vacuum of space and can run for years on end, the eventual velocity that this low thrust can impa

if you know what the trash looks like on x-ray, then you could try searching for it... I think they are more interested in the rocks below the trash tho, so the camera's are probably designed to see that and not the surface (with the trash).

Smart's map should provide that data and show if scientists are right in believing that the Moon coalesced from a vast ring of debris generated when an ancient planet the size of Mars destroyed itself after crashing into Earth. Understanding the origins of the Moon will therefore give insights into the nature of our planet.

Doesn't this mean earth should have some huge dent in it, and not be so round? Look at the sizes of Mars and Earth [nasa.gov]. Are you surprised earth is still here after a crash of that magnitude? I am. Maybe earth was a lot bigger before a Mars-like planet destroyed itself crashing into earth, but then I go back to my question about the roundness of the earth.

Maybe someone more knowledgeable wants to talk about that. The article doesn't go into any great detail on that, which causes a lot of questions to be raised.

For anybody who is interested, here's a theory [uc.edu] (bottom of the page): "one theory says the moon formed when a big, molten chunk of crust was knocked/blown off from the rest of the planet". And much more info [google.ca] about it.

Any planet (or asteroid, or whatever) above a certain mass will become spherical under its own gravity. I don't have the numbers at hand, but it is surprisingly low, if I recall correctly it would have to be roughly 30 miles wide to have enough mass.

I saw something on the Science Channel about this, and the illustration they had involved the two planets hitting each other and swirling around a bit. Since the earth is mostly moltent rock, I think during the collision, a lot of that came in to fill it in. Plus this was a long long time ago, so the Earth could have covered it up by now.
Not the best explaination I know, I can't say I understand it all myself.

There was once a theory that the Pacific Ocean was the hole left when the moon was pulled out of the Earth, but...

There is a reason why small objects, like asteroids, are often irregular in shape, while large objects, like planets, tend to be nearly spherical. All parts of an object are attracted to each other by gravity, this tends to pull the object into a spherical shape. Above a certain size (which depends on the materials involved) the object is not strong enough to maintain its shape and collapses

Beyond a certain size, gravity pulls things into a spherical shape. The immense pressure makes the insides molten and irregular structures eventually sink down in. Mars has Mons Olympus, the tallest volcano in the solar system, this is because Mars is smaller and has less gravity than Earth. The larger the planet the more regular it has to be. Asteroids can be highly irregular because they haven't the size and gravity to collapse them into spheres.

The mountains on Earth may appear huge to us insects on the surface, but from a distance the earth appears as smooth as a billiard ball.

Ironically this event was so big, that unlike latter smaller hits, all evidence in the way of dents will be gone as the entire globe virtually liquefied and coalesced again. Though I wouldn't rule out some exotic mass distributions that might lend evidence of it.

I no geologist or seismologist, but they believe the core is mostly iron of a taffy like consistency. The heat to keep it so comes from 2 sources: 1. The original condensation of the planet (miles of mantel and crust are good insulators keeping the heat in for billions of years). 2. The decay of heavy elements like uranium, which would offset the slow cooling process, mentioned in one.

Recent speculation is that the very center has a high ratio of Uranium, enough so that the pressure actually creates a self-sustaining natural nuclear reactor. When it gets too hot it diffuses and shuts down, only to coalesce and restart again (never a big boom). This starting and stopping of the nuclear processes at the Earth's core may be responsible for our planets large magnetic field, and occasional shut downs and reversals of the magnetic field as this nuclear process fluctuates.

You're right that the center would be weightless, but under more pressure than we can possible create in the lab with the best diamond anvils. It only takes a few miles of crust to crush carbon to diamonds, and here we are talking 8,000 miles of rock pressing down. Though the rock (iron) at the center isn't adding any additional pressure, it has thousands of miles of rock above it that is. Quite the hellish place.

BTW, I don't know how I typed Biq in my rirst post
when I meant to type Big (no one seems to have noticed)

You're confusing the Oklo natural reactor which occurred near the surface in Africa with the Earth's core. These are entirely separate. The core is iron, not uranium (for the most part). While radioactive materials are responsible for keeping the Earth's interior warm, there's no "reactor" involved (unstable isotopes really don't need help to decay).

Not confusing anything. I'm aware of the Oklo natural reactor, but I speak of a much more recent (and controversial) theory. When first proposed it was dismissed as total bunk, but it has gained support over the last five years. Much more respectable than cold-fusion, but hard to put odds on whether it will prove out or not. It is the inspiration for the movie "The Core"

Here is a link to a Discovery article [discover.com]
Nuclear Planet
Is there a five-mile-wide ball of hellaciously hot uranium seething at the cent

You're right. Its hard to find scientists who support the idea though. Mostly because its a solution looking for a problem. Field reversals have already been seen in computer models of the dynamo theory.

Recent speculation is that the very center has a high ratio of Uranium, enough so that the pressure actually creates a self-sustaining natural nuclear reactor.

I find this implausible. Surely SNO and Kamiokande should detect a massive neutrino flux from such a large reaction? Kamiokande can pick up neutrino fluxes from the nuclear reactors across east Asia [tohoku.ac.jp], so the flux from a monster reaction in the Earth's core should also be detectable.

Take a look at the moon. Those dark spots are the sites of enormous ancient impacts. They may have been holes briefly, but they then filled up with lakes of lava. As far as the Earth goes, the impact was so devastating that the outer layers of the Earth had to reform by falling back down.

The following contains some links to mostly non-technical explanations of planetary roundness. I'd like to point out that part of this explanation [sciam.com], by "Derek Sears, professor of cosmochemistry at the University of Arkansas and editor of the journal Meteoritics and Planetary Science," is wrong. He says "Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it." But this is a circular argument (pardon the pun). Generally a non-spherically symmetric distribution of matter doesn't have a gravitational field that acts as if it originates from the center of the body (the "center" being the center of mass). Spherically symmetric mass distributions do have this special property, so what Sears really implied is that planets that are already round will have gravitational fields that point towards the object's center of mass. This does absolutely nothing to address cases of objects that deviate from perfect roundness, i.e. all celestial bodies.
This explanation [astronomycafe.net] by Dr. Sten Odenwald suffers from the same argument, and there's even a hint of it here [nasa.gov]. Nonetheless, these explanations are approximately true, and require bizarre shapes to break them.

For example, imagine a homogenous, perfectly shaped doughnut (a torus with a circular cross section). At the center of the doughnut hole we'd feel no gravitational field at all (a perfectly balanced tug-of-war). But deviate from the exact center just a tiny amount, and the closer side of the doughnut becomes more attractive than the other. One suddenly experiences a gravitational field that points away from the center of mass.

For example, imagine a homogenous, perfectly shaped doughnut (a torus with a circular cross section). At the center of the doughnut hole we'd feel no gravitational field at all (a perfectly balanced tug-of-war). But deviate from the exact center just a tiny amount, and the closer side of the doughnut becomes more attractive than the other. One suddenly experiences a gravitational field that points away from the center of mass.

Do you have a link to a proof of this? I know that this is not the case for a

I don't know the deep maths (didn't do anything like that in my maths degree...) but one issue occurs to me: are objects with very different gravitational fields stable? If a roughly spherical shape is deformed (plastic flow due to gravity, impact, fragmentation, accretion, whatever), then its gravitational field will maintain a roughly spherical shape; but is that true of toroidal objects, for example?

I wouldn't be surprised if the majority of shapes with radically non-spherical gravitational fields are

Scientists believe nuclear-powered ion-drives are their only real hope of exploring deep space, and vigorously support their development. Not surprisingly, anti-nuclear protesters, like the Global Network Against Weapons and Nuclear Power in Space, have pledged opposition.

Too much time on Slashdot. Any group that starts GNA... automaticly activates my mental filters. (No great loss in the case of Huggers In Space.)

Quoting from the article,
"We have shown that even a small ion engine like Smart's can get us across space. Now we are planning to build space telescopes and robot probes to planets such as Mercury, using bigger and more powerful ion engines. These will take years off space-travel times. Instead of decades-long missions, we will take only a couple of years to cross space for future projects."

But,
"Ion engines need electricity and only solar panels can provide enough at present. So ion engine missions will be restricted to planets and moons near the Sun."

So the solution to deep space exploration is nuclear-powered ion-drives and NASA is working on it.

Can any astrogeologists explain why the assumption is that the Moon is a result of debris from a two-planet crash as opposed to regular accretion of debris the way moons were formed on the other planets?
Billions of years ago, each planet was a gradually coalescing disc, and the jovian planets still have evidence of this in their rings. The closer planets instead have moons - I assume because the Sun's gravity is stronger at our closer distance from the Sun, and caused the rings of the closer planets to agg

It's really freaking big. Mercury and Venus don't have any moons, and the moons of Mars appear to be captured asteroids...relatively tiny rocks not big enough to form themselves into spheres. The Earth-Moon system is nearly a double planet. Of the inner planets, Earth is the only one with a decent moon...and it's a monster compared to the planet.

In any case, the planets you see now are just the ones that stayed in the system. Material didn't just cleanly accrete directly into the existing bodies. Most of t

unlike in the 60s and 70s, they are using ion engines for this mission, which can run of solar power.they give less trust/second, but they can keep burning for allot longer, since the sun gives a constant supply of fuel (in the form of electricity from solar panels).so you've got a smaller probe, which means easier to get into orbit from where it can fly on it's own power, so even tho it takes longer to get where you want, it will be cheaper to get it into orbit.

btw, they are planning on bigger engines in the future, so hopefully they will go faster someday.

Ion engines still use propellant (typically Xenon, but I haven't looked at what this particular mission is doing), they simply accelerate the propellant using electrostatic fields (in the case of ion engines) instead of chemical combustion. The key to ion engines isn't so much the solar power, as the fact that they have a much greater specific impulse (a rocket engineering term that relates to the efficiency with which propellant is used). Where a chemical engine may top out at ~400 s of specific impulse, ion engines have hit around 4000 s, or an order of magnitude greater propellant efficiency. That translates into a much smaller amount of propellant to do the same mission.

The tradeoff is betwen the extra time it takes to get to the destination (due to the low thrust of an ion engine), and the reduced cost created by being able to launch a much smaller amount of mass into space in order to do the mission.

btw, they are planning on bigger engines in the future, so hopefully they will go faster someday.

The issue with ion enginer thrust is not so much size, as it is power. The thrust you get is directly proportional to the amount of power you can generate. If you're using solar arrays, then you're limited to something between 15-20 kW (the Boeing 702 has solar arrays that produce ~15 kW at end-of-life).

SMART-1 has been making bigger and bigger orbits around the earth, because of the smaller thrust explained above. It goes faster and faster, and because the craft gets further from the earth, it becomes easier to get into a bigger orbit. So at first the orbits became larger very slowly, but the last months it has grown faster than ever before.

The last months the orbit was also synchronized with the moon. The highest part of SMART-1's orbit coincided with the lowest point of the moon's orbit. This helps the craft to get an extra boost every month. Take a look at a graph of the orbit here [esa.int].

Oh, and they do have normal propellant onboard, there's some 70kg left iirc. I think it was installed in case the ion engine failed, but I'm not sure of that. It could also be to correct the initial orbit if the launcher would have placed it in a wrong one. Anyway I *hope* it will be used to attempt a soft landing after the mission is over.

In all seriousness it's nice to see some other serious large-scale attempts being made by countries other than the US and Russia. As with all things scientific, the more head working together the more we all learn.

This is hardly a large scale attempt... It is an interesting project and new stuff about Ion propulsion will be learned, but a huge project it is not. If you want large-scale European space efforts, look up "Rosetta".